The United States is in the midst of an obesity epidemic and innovative approaches to address this problem are urgently needed. Although traditional behavioral treatment is the most empirically validated approach for weight loss, it typically results in poor maintenance of weight control behaviors, and within three to five years most participants in these programs will have regained the weight they originally lost. Standard behavioral weight loss treatment might be improved by incorporating components that (a) bolster participants'commitment to behavior change, (b) build distress tolerance skills and (c) promote mindful awareness of eating behaviors and goals. Such components are well- represented within newer models of behavior therapies that incorporate principles and technologies of mindfulness, experiential awareness, values for changing behavior, acceptance of distressing internal experiences, and willingness to tolerate distress in the service of valued behavior change. The primary goal of the proposed project is to determine if an innovative behavioral weight loss program that incorporates new developments from the field of behavior therapy produces superior weight control than standard behavioral treatment. A sample of 128 adults with a BMI of 27 to 40 kg/m2 will be recruited via media advertisements in the greater Philadelphia community to participate in this study. Participants will be randomly assigned to one of two conditions: standard behavioral treatment (SBT) or acceptance-based behavioral treatment (ABBT). Each treatment will be delivered in a group format on a weekly, and then bi-weekly, basis. Treatment will last 40 weeks: weeks 1-20 focus on weight loss, while weeks 21-40 partially shift focus to weight loss maintenance. Assessments will be completed at baseline, 10 weeks, 20 weeks (i.e., end of weight loss treatment), 40 weeks (i.e., end of weight loss maintenance treatment), and at 6-month follow-up. PUBLIC HEALTH RELEVANCE: Obesity is reaching epidemic proportions. Standard behavioral treatment for obesity effectively induces weight loss, but long-term maintenance of weight loss remains challenging. The primary aim of the proposed study is to develop and conduct a preliminary evaluation of an innovative, acceptance-based behavioral weight loss treatment that is designed to promote long-term adherence to healthy eating and physical activity behaviors and thereby improve weight loss maintenance.